1. Field of the Invention
The present invention relates to a driving device of an object lens in an optical disk drive or a magneto-optic disk drive, an optical pickup, and an optical disk drive.
2. Description of the Related Art
In an optical disk drive, a laser beam is emitted onto an optical disk, which acts as a recording medium, and information recorded in the optical disk is extracted from light reflected by the optical disk. An object lens driving device installed in the optical disk drive controls the object lens so that the object lens follows up wobble or eccentricity of the recording medium so as to drive the object lens in a focusing direction and a tracking direction and form a light spot on a recording surface of the recording medium.
When dealing with data of a large size, it is desirable that recording and reproduction be carried out at high speed, and for this purpose, it is necessary to drive the recording medium to rotate at high speed. However, when driving a recording medium having wobble or eccentricity to rotate at high speed, because the acceleration of the recording medium increases, and in order to ensure that the object lens to follows the recording medium precisely, it is necessary to use an object lens driving device able to generate a large thrust force.
On the other hand, in recent years and continuing, in order to further increase the recording density of the optical disk, it is necessary to form a small light spot on the recording medium. For this purpose, it is suggested to the increase NA (Numerical Aperture) of the object lens or decrease the wavelength of the incident laser light. When the NA of the object lens is increased or the wavelength of the incident laser light is decreased, however, the optical axis of the object lens turns out to deviate from the normal direction of the recording medium, and this may induce chromatic aberration easily. Due to this, the quality of the light spot may degrade, and the recording and reproduction quality may decline. Therefore, it is required to improve the accuracy of tilts of the recording medium and the object lens in order to increase the recording density.
Specifically, if the object lens driving device drives operations of the object lens only in the focusing direction and the tracking direction, which is the so-called “two-axis object lens driving device”, it is necessary to reduce the tilts of the object lens during focusing and tracking operations.
In recent years and continuing, higher precision is required, and an optical disk drive has been proposed that is able to positively correct the tilts of the recording medium and the object lens.
There are several methods of correcting the tilts of the recording medium and the object lens. Particularly, an object lens driving device has been proposed which carries out tilt driving by rendering a movable unit of the object lens driving device to follow up the tilt of the recording medium. The object lens is included in the movable unit. This object lens driving device has a low cost and a small size, hence occupying only a small space. In addition, since the movable unit is light-weight, it can follow up the tilt of a high-speed rotating recording medium.
However, even for such an object lens driving device capable of tilt driving, if the tilts arising in focusing and tracking operations are large, servo-control in the device is liable to be out of phase. Hence, it is required to reduce the tilts arising in the focusing and tracking operations.
Below, a description is made of an example of the object lens driving device of the related art.
FIG. 23 is a cross-sectional view illustrating a schematic configuration of a two-axis object lens driving device as a first example of the related art.
As illustrated in FIG. 23, in a two-axis object lens driving device 100, a movable unit 102 which includes an object lens 101 is supported at one side by four rod-like elastic supporting members 104 in the tangential direction on a fixed portion 103.
The two-axis object lens driving device 100 has a focus driving coil 105, which is a cylindrical coil with a focusing direction to be a winding axis. The magnetic flux of a magnet 107 penetrates, in the tangential direction, a portion of the focus driving coil 105 in which an electric current flows in the tracking direction.
The two-axis object lens driving device 100 further has two tracking coils 106, each of which is a planar coil with the tangential direction to be a winding axis. The two tracking coils 106 are arranged in a line in the tracking direction. Similarly, the magnetic flux of the magnet 107 penetrates a side of each of the tracking coils 106, in which an electric current flows in the focusing direction.
When electric currents flow in the respective driving coils, the two-axis object lens driving device 100 drives the object lens so as to perform focusing and tracking operations.
In this example, when offsets are provided in the focusing direction and the tracking direction, because of a density distribution of the magnetic flux of the magnet 107, the position of the center of a thrust force along the focusing direction deviates from its normal position.
FIGS. 24A through 24C are cross-sectional views showing a schematic configuration of a two-axis object lens driving device as a second example of the related art.
As illustrated in FIGS. 24A through 24C, an object lens holding member 112 for holding an object lens 111 is elastically supported by four rod-like elastic supporting members, such as four wire springs 114, which extend from a stem 113. An object lens 111 is fixed at an end of the object lens holding member 112. In addition, a driving coil assembly 115 is stacked and fixed on the object lens holding member 112, being positioned beside the object lens 111. A prism 117 is mounted on a plate member which is integrated with the stem 113.
Next, with appropriate spaces being provided in the axial direction of the wire springs 114 of the object lens holding member 112, two sets of yokes 118, 119 and driving magnets 120, 121 are mounted on a plate member integrated with the stem 113 to face each of the driving coil assemblies 115. Each of the driving magnets 120, 121 is square-shaped and is divided into four divisions along cross-shaped magnetization boundaries for magnetization. The direction of magnetization is perpendicular to a plane including the focusing direction and the tracking direction, and the magnetization directions of adjacent divisions are opposite to each other.
Each of the driving coil assemblies 115 is constituted by a combination of two focus driving coils 122 and two tracking driving coils 123. The two focus driving coils 122 each including a planar coil are provided on two sides of the magnetization boundary in the focusing direction of the driving magnets 120, 121. On the other hand, the two tracking driving coils 123 each including a planar coil are provided on two sides of the magnetization boundaries in the tracking direction of the driving magnets 120, 121. The two tracking driving coils 123 are mounted on the object lens holding member 112 across the magnetization boundaries in the focusing direction. The driving magnets 120, 121 are provided to sandwich the focus driving coils 122 and the tracking driving coils 123 with respective portions of the driving magnets 120, 121 facing each other having the same magnetization direction. In this way, the focus driving coils 122, the tracking driving coils 123, and the driving magnets 120, 121 constitute a driving motor 124.
In the second example of a two-axis object lens driving device in the related art, a movable unit which includes an object lens is supported at one side by rod-like elastic supporting members in a tangential direction on a fixed portion.
In this structure, in order to reduce the moment generated by the focusing coils, centers of the driving coils are set to be approximately in agreement with centers of the corresponding magnetic flux density distributions, or intervals of the centers of the driving coils are set to be more or less greater than intervals of the centers of the magnetic flux density distributions. With such settings, the moment generated by the focusing coils are reduced, and tilts arising in the focusing and tracking operations are reduced.
FIG. 25 is a perspective view illustrating a schematic configuration of an object lens driving device as a third example of the related art.
FIG. 26 is a perspective view illustrating a configuration of a driving motor in the object lens driving device in FIG. 25.
Illustrated in FIG. 25 and FIG. 26 are an object lens 131, an object lens holding member 132, focus driving coils 133, a track driving coil 134, wire springs 135, fixed members 136, a base 137, driving magnets 138, elastic substrate 139, and a radial (rotational) tilt coil 140.
The object lens driving device illustrated in FIG. 25 and FIG. 26 is a four-axis object lens driving device, which is able to tilt the object lens 131 in response to angular deviations of the object lens 131 and a recording medium.
In the object lens driving devices illustrated in FIG. 23 and FIGS. 24A through 24C, which utilize four wire springs arranged in parallel, the supporting rigidity in the tangential tilt direction ends up increasing greatly. In contrast, in the object lens driving devices illustrated in FIG. 25 and FIG. 26, eight rod-like elastic supporting members are arranged in the same plane. Due to this, a tension and compression stress along the axial direction of rod-like elastic supporting member does not appear during operations in the tangential direction. By such a structure, similar moving properties can be obtained in the tangential tilt direction and in the radial tilt direction.
For example, Japanese Laid-Open Patent Application No. 2003-91844 discloses a technique in the related field.
In the third example, a movable unit, which includes the object lens holding member 132, the focus driving coils 133, the track driving coil 134, and so on, is supported from two sides in the tangential directions. Due to this, moments generated when driving focusing operations are cancelled out on the two sides, and thus, the tangential tilt does not take place, theoretically.
On the other hand, when performing focusing and tracking at the same time, there arises a problem in that radial tilt (rotation) may take place. Specifically, when driving in the focus+direction and the track+direction, the movable unit tilts in the radial direction. Furthermore, the magnitude of the radial tilt changes along with changes of magnitudes and directions of the focusing driving operation and the tracking driving operation.
FIG. 27 presents typical tilts arising in the supporting system in the object lens driving device shown in FIG. 25.
Tilts shown in FIG. 27 are caused by deviation of a supporting center from the center of the movable unit during focusing movement and tracking movement.
FIGS. 28A and 28B are views for showing the reason of the tilts arising in the supporting system in the object lens driving device shown in FIG. 25.
In FIGS. 28A and 28B, from the time when the movable unit is at a center valve position (FIG. 28A), the focusing movement at the time of movement for focusing and tracking (FIG. 28B) is taken as an action radius and moment generated by a tracking driving force. With such relationships, the reasons of the tilts in FIG. 27 can be easily understood.
FIGS. 28A and 28B illustrate driving in the focus+direction and the tracking+direction, and the movable unit tilts in the radial direction.
The tilts shown in FIG. 27 are not as large as those in the object lens driving devices in FIG. 23 and FIGS. 24A through 24C, in which a movable unit is supported at one side in the tangential direction. However, in recent years and continuing, in order to meet the requirements of higher precision, it becomes necessary to reduce tilts arising in focusing and tracking operations.